This invention relates to a DC motor and, more particularly, to such a motor wherein, upon energization of the rotor or stator assembly thereof, results in a torque of a predetermined rotational direction which is capable of extending over an electrical angle greater than 180.degree., and wherein such torque exhibits relatively small ripple.
Various types of brushless DC motors are known, such as a 2-pole alternate phase motor, a 2-pole 3-phase motor, a bi-directional 2-phase motor, a 4-pole motor, and the like. In a 2-pole alternate phase motor, a single pair of magnetic poles is provided, usually formed of permanent magnet north and south pole pieces, and two alternately energized coils also are provided. Either the permanent magnet pole pieces or the coils may constitute the rotor assembly, and the rotor may be disposed either within the stator assembly or in circumscribing relation with respect thereto. Thus, each pole piece extends for an electrical angle of 180.degree., and each coil likewise subtends an arc of 180 electrical degrees.
In a typical 2-pole alternate phase motor, each coil includes conductor segments for carrying current in directions which are normal to the magnetic flux generated by the permanent magnet pole pieces. In accordance with Fleming's rule, also known as the left-hand rule, torque is produced in a direction perpendicular both to the direction of flux and the direction of current flow. The coils are wound on a cylindrical surface so that theconductor segments include a first current path portion for carrying current in a first direction and a second current path portion for carrying current in a second, opposite direction, these current path portions being separated by 180 electrical degrees. If one coil is energized at the moment that the first current path portion enters the region of magnetic flux having, for example, a north polarity, a rotational torque is produced so as to drive the rotor assembly in a given direction. This coil then is deenergized at the moment that the first current path portion leaves the region of magnetic flux having north polarity, and the other coil then is energized. Thus, each coil is energized only when its first current path portion first enters the region of magnetic flux of given polarity. Consequently, the resultant torque is provided with significant ripple, which may not be desirable. Furthermore, if the motor comes to rest with the first current path portion of each coil disposed in the region of magnetic flux polarity transition, it is necessary to provide auxiliary starting means for subsequently starting motor rotation.
In a typical 2-pole 3-phase motor, magnetic flux is generated by permanent magnet north and south pole pieces. However, the coil structure here is formed of three coils, as opposed to the two coils described in the aforementioned 2-pole alternate phase motor. Each coil is wound on a cylindrical surface and includes first and second current path portions which are separated from each other by 180 electrical degrees. However, the first current path portion of one coil is separated by 120.degree. from the first current path portion of the next adjacent coil. Rotational torque of a given direction is produced when the first current path portion of a coil has advanced by a given electrical angle into the region of magnetic flux of predetermined polarity. This predetermined angle generally is about 30 electrical degrees. After one coil has been energized for a duration corresponding to 120 electrical degrees, the first current path portion of the next coil will have been advanced by an angle of 30 electrical degrees into the region of magnetic flux of predetermined polarity. At that time, the first coil is de-energized and the next coil is energized. Consequently, all three coils are energized in sequence, resulting in an overall torque whose ripple is substantially reduced from the ripple attending the aforedescribed 2-pole alternate phase motor. In addition, because of the particular dimensions of each permanent magnet pole piece, the angle subtended by each coil and the hase displacement of the respective coils, the problem of the motor coming to rest at a zero torque location, mentioned above with respect to the 2-pole alternate phase motor, is avoided. That is, auxiliary starting means is not necessary to impart a starting rotation to the motor regardless of the position at which it comes to rest.
However, one undesirable feature of the 2-pole 3-phase motor is the need for requiring three position sensing elements for detecting the relative position of each coil with respect to the permanent magnet pole pieces. These three position sensing elements are needed so as to control the selective energization of each coil. The locations of these position sensing elements must be carefully established during the assembly operation in the construction of the DC motor such that each position sensing element is properly aligned with its associated coil. This tends to increase the cost of assembly, and thus the overall cost of such a motor. Furthermore, if the position sensing elements are packaged in module form, such a packaged module can be used only with a 2-pole 3-phase motor of corresponding diameter. A different package module must be used for different diameter motors. Still further, since three coils are provided, three separate switching circuits must be used in order to selectively energize the respective coils. Thus, although the torque characteristics of the 2-pole 3-phase motor are improved over the torque characteristics of the 2-pole alternate phase motor, this improvement is achieved at a significantly increased cost of the motor.
In the 4-pole 2-phase motor, four separate energizations, or current change-over operations, must be carried out over an angle of 360 electrical degrees. This requires two separate position sensing elements and four separate switching circuits. The bi-directional 4-pole motor similarly requires two position sensing elements, but also must be provided with four separate switching circuits for each rotational direction of the motor. Hence, this bi-directional motor must be provided with eight separate switching circuits. It is appreciated that such 4-pole motors are significantly more expensive than the relatively simple 2-pole alternate phase motor.